Absolutely Amazing, Well worth the money
As you might expect, the graphics are very retro, but that creates an amazing look and feel to the overall performance of the game. The controls are mainly simple and easy to pick up, and it is very addictive.
Graphics: As mentioned above, the graphics aren't stunning but they are enough to keep the player wanting more. It shows that true gaming isn't about quality of graphics but game design and ideas. They are simple but effective.
Difficulty: I think that this game is very fun, but can get EXTREMELY frustrating at times! Being a perfectionist, I found it irritating to spend about 20 mins to try and get to a hidden exit, but the time it takes to improve is quite solid and steady. This isn't helped by the hidden exits though, which you might occasionally find by accident and unlock a super-hard route.
Music: Brilliant. Lovely music, often very scenic and fitting to the level. As goes with most retro games, it gives a nostalgic feel to the gameplay and suits the Mario and Luigi theme.
I conclude that this is a very fun game, and expect to be addicted. This is a must-have for perfectionists since you'll be keen to go back and find everything, but for people who couldn't care less, you could be finding yourself finishing this surprisingly quickly!
Also, expect to see lots of game over screens.